The semiconductor industry often uses far field optical imaging tools such as microscopes or scatterometers for wafer inspection and metrology. Such tools typically couple light from a source to a target through an objective lens. Light scattered from the target is coupled back through the objective to an imaging detector via an optical column. Such far-field optical tools can operate quite quickly and can obtain three-dimensional information about a target.
Another class of imaging tools includes electron microscopes, such as the scanning electron microscope (SEM). The SEM can form an image of a target using an electron beam instead of light. The SEM operates in a similar manner to an optical microscope. Instead of an optical beam, an electron beam is coupled to the target through an objective and secondary electrons from the target are coupled back through the objective to a detector through an electron optical column.
Yet another class of tools used for inspection and metrology is known as scanning probe tools. Such tools use a small probe that is mechanically scanned across a target. Atomic scale interactions between the target and the probe can be detected and amplified and converted to an image as the probe is scanned across the target.
As the components of semiconductor devices shrink to smaller and smaller sizes, presently on the order of a few nanometers, the ability to improve metrology performance, productivity, and device correlation becomes critically important. To determine the alignment of features formed in one layer with respect to features formed in a previous or subsequent layer, metrology targets are often formed in one or more of the layers. The metrology targets facilitate measurement of the dimensions of features and alignment, e.g., overlay between the features, using a metrology tool such as microscope or scatterometer or critical dimension (CD) with SEM or scatterometer.
One of the main obstacles in achieving these metrology requirements with optical based technologies is the large wavelength of the relevant spectral bands and the consequent limited resolution. These limitations are relevant to common far field technologies. Conventional optical metrology methods are based on propagating optical waves and as such are limited by wavelength range resolutions.
Unfortunately, in an optical system operating in a far-field mode, the spatial resolution is diffraction limited. The diffraction limit in traditional optics is imposed by the use of propagating waves in the imaging process. Generally, the lateral spatial resolution d is given by the Rayleigh criterion:
  d  =            1.22      ⁢      λ        NA  where λ is the wavelength of the light being imaged and NA (the numerical aperture) is equal to n sin θ, where θ is half of the angular aperture on the object side and n is the refractive index of the medium above the object.
One way of overcoming the diffraction limit for an optical system is to use an electron beam system, such as a scanning electron microscope (SEM). Because the SEM uses electrons instead of light SEM can measure much smaller features than can be measured with optical systems. However, a key difference between a conventional SEM and an optical microscope is that an SEM must operate with the target under vacuum. This can slow down the rate at which an SEM can inspect or measure different wafers and also makes an SEM system more expensive and complex. In addition, SEM typically requires that the target be made of an electrically conductive material. Non-conductive materials can be coated with a thin layer of conductive material, e.g., gold, prior to SEM measurements. Unfortunately, such coatings may alter the target, e.g., by obscuring very small scale features.
Furthermore, scanning probe technologies, such as Atomic Force Microscope (AFM), Scanning Tunneling Microscope (STM) and electronic microscope, are best suited for measuring two-dimensional information and have limited ability to provide three-dimensional information. In addition, the measurement and imaging process with scanning probe tools is slow.
The diffraction limit can be overcome by techniques that include near-field radiation (sometimes referred to as evanescent waves) in the detection process. One example of a technique for overcoming the diffraction limit using near-field radiation is described in commonly-assigned U.S. patent application Ser. No. 12/256,324, filed Oct. 22, 2008 entitled SYSTEMS AND METHODS FOR NEAR-FIELD HETERODYNE SPECTROSCOPY, by Guorong V. Zhuang, John Fielden, and Christopher F. Bevis, the entire contents of which are incorporated herein by reference. In this system a target is probed using a probe beam and a modulated pump beam. A near-field generation device receives the modulated pump beam, generates a modulated near-field beam, and directs the modulated near-field beam to a point on the sample that is probed with the probe beam. A reflected probe beam is detected, demodulated, and analyzed. However, this system detects the reflected probe beam in far field as opposed to the near field.
It is within this context that embodiments of the present invention arise.